The present invention relates to an extrusion device for use in blow molding machines and more particularly to a new and improved extrusion head having a plunger and a distributor head for feeding plastic flowable material into an accumulator for blow molding.
Extrusion-blow molding is done as a continuous extrusion process or as an intermittent extrusion process. The continuous extrusion process is best suited for PVC (polyvinyl chloride) and other heat-sensitive resins because the gentle uninterrupted flow reduces the chance for resin thermal degradation. In such continuous molding, the molding process used is either shuttle or rotary to accommodate the continuous flow. The intermittent extrusion process is best used for polyolefin and other non-heat sensitive resins and permits the use of simpler molding mechanisms. The intermittent extrusion machines fall into three categories: reciprocating screw, ram accumulator, and an extrusion accumulator head systems. The reciprocating screw allows accumulation of the melt in front of itself and thence the screw is rapidly pushed forward, forcing the melt through a die head to form a parison. In this instance, the melt remaining in the screw during the forward movement thereof is subjected to heat and can cause problems of degradation if tried with PVC or other heat sensitive thermoplastic materials. The ram accumulator allows the accumulation of the melt in an auxiliary cylinder and is extruded by a special ram. In this case, the disadvantage is that the melt that enters the accumulator first is the last to be extruded. The melt history for this system is not uniform. The accumulator head serves as an extruding die head type having a movable tubular plunger which moves as the accumulator chamber is filled directly from the extruder which has a first in/first out melt flow path. In a typical accumulator head, the melt enters from one side and divides as it flows around the plunger head, reknitting on the opposite side as it fills the accumulator chamber. One problem encountered with this process and the use of the equipment available is weld or knit lines. From a rheologically correct design of melt distribution, it appears and shows a thickness distribution that because of orientation in the flow lines (knit-lines) generated where the divided melt stream comes together, it stretches more here than elsewhere and this leads to thin spots in the blown article. This stretching becomes more apparent when increasing the degree to which the article is increased in size.
Reference is made to U.S. Pat. No. 4,422,838 which discloses an extrusion head with a body portion supporting a movable diverter plunger that is located therein, such that as a melt flows into the body portion through an inlet aperture, the melt flows via a groove into a cavity or flared portion to move around a throttling lower edge. The gap between the cavity and the surrounding wall is essentially uniform as seen in FIG. 4. Herein melt will flow up the groove as the plunger rises, which melt will not be extruded until after an accumulation. In addition, the dimension of the gap of the longitudinal flow channel is essentially constant from the inlet end to the terminal end which results into a non-uniform flow beyond the tapered shoulder or throttling lower edge. Such restrictive throttling lower edge results in excessive shearing of the melt and causes excessive localized heating which would cause thermal degradation.
One solution attempted was to use dual or coaxially spaced melts that are also concentric. However, even in this instance, there is the additional problem of the hot melt remaining in the longitudinal groove where degradation can begin. The object of the present invention is to minimize any residence time in the accumulator, thereby enhancing its ability to process heat-sensitive materials. In addition, a uniform wall thickness in the parison melt is obtained with this new and improved melt feeding system which assures uniform melt distribution around the entire circumference of the parison without the creation of thin spots.